William Astbury discovered that the x$-$ray diffraction pattern of wool shows a repeating structural unit spaced about $5\mathrm{.}2$ along the length of the wool fiber. When he steamed and stretched the wool, the x$-$ray pattern showed a new repeating structural unit at a spacing of $7\mathrm{.}0.$ Steaming and stretching the wool and then letting it shrink gave an x$-$ray pattern consistent with the original spacing of about $5\mathrm{.}2.$ Although these observations provided important clues to the molecular structure of wool, Astbury was unable to interpret them at the time.

Given our current understanding of the structure of wool, interpret Astbury$$s observations.

When wool sweaters or socks are washed in hot water or heated in a dryer, they shrink. Silk, on the other hand, does not shrink under the same conditions. Explain.